Allow access to the virtual counter in the ARM generic timer

[andrew]

Summary

Allow access to the virtual counter in the ARM generic timer. The SoC
may not allow access to the physical counter from non-secure PL1. In
this case we will need to access the virtual counter which is always
available.

[andrew]

Need to fix a comment containing a register name.

sys/arm/arm/generic_timer.c

157 ↗

(On Diff #1052)

This should be cntv_tval

[andrew]

Add the virtual timer irq handler.

[andrew]

Update gic registers

[andrew]

Fix a comment to have the correct register name

[andrew]

Closed by commit rS271189 (authored by @andrew).